optimisation vs improvement

optimisation and improvement both are nouns.

Word	Noun	Adjective	Verb	Adverb
optimisation	Yes	No	No	No
improvement	Yes	No	No	No

As nouns, improvement is a hypernym of optimisation; that is, improvement is a word with a broader meaning than optimisation:

optimisation (noun)	improvement (noun)
the act of rendering optimal	the act of improving something
	a change for the better; progress in development
	a condition superior to an earlier condition